Follow-up from "Update Entity to expose mergeAndReset method of internal object"

The following discussion from !1253 (merged) should be addressed:

• @dcraik started a discussion: (+10 comments)

  Hi @clemenci unfortunately this doesn't work because in the vast majority of cases the Entity is created when MonitoringHub::registerEntity is called from the constructor of the base class BufferableCounter - so many incompatible Entity objects will share an identical typeindex. Additionally, the Entity created when a Counter is constructed will not match the typeindex of an Entity created from exactly the same Counter after the fact (when the derived class has finished being constructed).